The present invention relates to improved electrodes particularly adapted for use as anodes in electrochemical process involving the electrolysis of brines.
A variety of materials have been tested and used as chlorine anodes in electrolytic cells. In the past, the material most commonly used for this purpose has been graphite. However, the problems associated with the use of graphite anodes are several. The chlorine overvoltage of graphite is relatively high, in comparison for example with the noble metals. Furthermore, in the corrosive media of an electrochemical cell graphite wears readily, resulting in substantial loss of graphite and the ultimate expense of replacement as well as continued maintenance problems resulting from the need for frequent adjustment of spacing between the anode and cathode as the graphite wears away. The use of noble metals and noble metal oxides as anode materials provides substantial advantages over the use of graphite. The electrical conductivity of the noble metals is substantially higher and the chlorine overvoltage substantially lower than that of graphite. In addition, the dimensional stability of the noble metals and noble metal oxides represents a substantial improvement over graphite. However, the use of noble metals as a major material of construction in anodes results in an economic disadvantage due to the excessively high cost of such materials.
Considerable effort has been expended in recent years in attempts to develop improved anode materials and structures utilizing the advantages of noble metals or noble metal oxides, while minimizing the amount of noble metals or noble metal oxides employed. A great amount of effort has been directed to the development of anodes having a high operative surface area of noble metal or noble metal oxide in comparison with the total quantity of the material employed. This may be done, for example, by employing the noble metal as a thin film or coating over an electrically conductive substrate. However, when it is attempted to minimize the aforementioned economic disadvantage of the noble metals by applying them in the form of very thin films over a metal substrate, it has been found that such very thin films are often porous. The result is an exposure of the substrate to the anode environment, through the pores in the outer layer. In addition, in normal use in an electrolytic cell, a small amount of wear, spalling or flaking off of portions of the noble metal or noble metal oxide is likely to occur, resulting in further exposure of the substrate. Many materials, otherwise suitable for use as a substrate are susceptible to chemical attack and rapid deterioration upon exposure to the anode environment. In an attempt to assure minimum deterioration of the substrate under such circumstances, anode manufacturers commonly utilize a valve metal such as titanium as the substrate material. Upon exposure to the anodic environment, titanium, as well as other valve metals, will form a surface layer of oxide which serves to protect the substrate from further chemical attack. The oxide thus formed, however, is not conductive and as a result the operative surface area of the anode is decreased.
In attempts to avoid the use of the expensive noble metals various other anode materials have been proposed for use as coatings over valve metal substrates. In U.S. Pat. No. 3,627,669, it is disclosed that mixtures of tin dioxide and antimony oxide can be formed as adherent coatings on a valve metal substrate to form an anode useful in electrochemical processes. In the electrolytic production of chlorine, alkali metal hydroxides, alkali metal chlorates and the like, anodes of this type provide the advantage of economy in the elimination of the use of expensive noble metals or noble metal oxides. In addition the tin oxide coating provides an effective protection for the substrate. However, the tin oxide compositions, although useful as anode materials and as a protective coating to prevent passivation of the valve metal substrate, nevertheless exhibit a chlorine overvoltage that is substantially higher than that of the noble metals or noble metal oxides. It has also been disclosed that noble metal oxides may be incorporated in coatings of a non-noble metal oxide. Thus, for example in U.S. Pat. Nos. 3,701,724 and 3,672,990, it is disclosed that anodes may be prepared which consist, for example of a valve metal substrate having a coating thereon which contains a mixture of a noble metal oxide such as ruthenium oxide, and a non-noble metal oxide, such as an oxide of tin, antimony, germanium, or silicon. Such anodes provide the electrocatalytic properties associated with the noble metal oxides while lessening the proportion of noble metal required. However, it has been found that when substantially lower amounts of the noble metal oxide are employed, for example, less than about 20 percent of the coating, the chlorine overvoltage is increased noticeably. It will be recognized that a continuing need exists for the development of anodes, materials and structures whereby the use of noble metals or noble metal oxides may be substantially minimized or eliminated.
Accordingly, it is an object of the present invention to provide improved electrodes for use as anodes in the electrolysis of aqueous solutions of ionizable chemical compounds, especially brines. It is a further object to provide such anodes wherein the amount of noble metal or noble metal oxide employed is substantially minimized or eliminated. It is a still further object to provide such anodes having an operative surface of noble metal or noble metal oxide and having improved efficiency and maintenance characteristics. It is an additional object to provide an improved method for the electrolysis of aqueous solutions of ionizable chemical compounds, especially brines.